A liquid crystal display apparatus has almost become an image display apparatus that is superior to a cathode ray tube because of (a) features such as small-footprint and power savings and (b) recent improvement in performance such as viewing angle, contrast, color reproducibility, and response speed. Thus, it is anticipated that such a liquid crystal display apparatus may be more often and widely applied to a monitor for a television set, office automation, etc. in future.
In general, when a liquid crystal cell receives a voltage, the major axis (director) of a liquid crystal material (liquid crystal molecule) in the liquid crystal cell is changed due to its dielectric anisotropy. Because the liquid crystal material has optical anisotropy, the polarization direction of the light that transmits through the liquid crystal cell is also changed in response to the change in the major axis. The amount of light that transmits through the liquid crystal cell is controlled, via a member in the liquid crystal cell such as a polarizing plate, in response to a voltage (an applied voltage) which is applied to the liquid crystal cell. This ensures that each pixel displays with a target gradation luminance so as to carry out the image display.
However, it takes some time for the liquid crystal material to respond to a change in the applied voltage, because the response speed of the liquid crystal material is slow. For instance, in the case of TN (Twisted Nematic), IPS (In-Plane-Switching), or VA (Vertically Aligned) in a liquid crystal display system (liquid crystal display mode) that has been widely used, the response speed of the liquid crystal material between slow gradations corresponds to 30 msec to 50 msec. Thus, it is not possible to realize the response speed that corresponds to 60 Hz (about 16.6 msec) of NTSC (National Television System Committee) signal or 50 Hz (about 20.0 msec) of PAL (Phase Alternation by Line). In order to meet the requirement of market expansion, higher performance appears to be necessary.
In view of the foregoing circumstances, a conventional liquid crystal display apparatus, in which the driving method for displaying the liquid crystal material is contrived so as to improve the response speed, has been studied and developed.
For instance, Japanese unexamined patent publication No. 10-39837 (publication date: Feb. 13, 1998) discloses a liquid crystal display apparatus adopting “overshoot-driving”. In such a liquid crystal display apparatus, a voltage greater than a corresponding voltage difference is applied during the change in gradation so as to rapidly move the liquid crystal material to a target gradation. In the overshoot-driving, look-up tables (LUTs) are prepared in advance in which gradation values (applied gradation values) to be applied to the liquid crystal material are set in association with respective start (current) and target (desired) gradations, and the voltage is applied in accordance with the LUT. The gradation values may be applied voltages that realize the applied gradation.
According to the arrangement of the publication, however, the LUTs would desirably be prepared by finding, in advance, applied voltages (applied voltages for all the gradations) corresponding to all the patterns of gradation change. This causes the problem that the capacity of the memory storing the LUTs becomes extremely large.
Further, according to the arrangement of the publication, it is sometimes impossible to appropriately carry out the overshoot-driving under some additional condition, such as temperature, of the liquid crystal display apparatus, thereby causing another problem that it is not possible to carry out natural and high-speed display.
More specifically, the response speed of the liquid crystal display apparatus remarkably changes in response to the change in the viscosity of the liquid crystal material due to the temperature change of the liquid crystal display apparatus. This results in that the overshoot-driving is not fully effective in lower temperatures due to the decreasing of the response speed of the liquid crystal material, when the LUTs are used in which the applied gradations are set at room temperatures. This causes the response speed not to be fully fast, so that the writing can not be in time. In contrast, the overshoot-driving becomes too strong at higher temperatures. This results in that the display is carried out in which white and black are excessively emphasized. This causes the display characteristics to be damaged.
In order to address the problem, a system is conceivable in which a plurality of LUTs is prepared for each temperature in advance and an optimum LUT is automatically selected as a target LUT to be used from among the LUTs in response to a temperature sensor so as to match for the temperature of the liquid crystal display apparatus. However, in view of the capacity of the memory storing the LUTs, it is actually difficult to prepare LUTs for all the gradations and for all the temperatures.
Furthermore, the foregoing explanation only deals with the case where the additional condition is temperature. There are other factors than temperature that cause the response speed of the liquid crystal display apparatus to change. Namely, the additional condition may include the thickness of the cell of display panel and the frequency of the image, for example. Thus, it is very difficult to prepare all the LUTs for the respective additional conditions.